[WIP] allow pr2.urdf.xacro to accept customized calibration values

[Smilels]

1. add calibration parameters in separate xacro files
2. in urdf/shoulder_v0/shoulder.urdf.xacro, we detect if parameters shoulder_pan_calibi, upper_arm_roll_calib equal to 0.
   If equal, use the default values. If not equal, use the passing values.

[v4hn]

@k-okada Please review and merge. I made sure this change is backward compatible and it provides a useful extension to easily set calibration values from external data (e.g. a yaml file). Long(er) story below.

Last week we noticed once again how much work it can be to update a calibrated PR2 urdf when the xacro files change. To finally resolve this after all this time, we now store the calibration tags in a yaml file and instantiate the macros to use the values instead of loading a generated and calibrated urdf.

In theory there is already another mechanism for this in the xacro files, but we decided to ignore it because

• it splits the calibration values into two separate computed qualities and our calibration procedure only outputs a single value
• They assume the left and right arm have the same calibration flags
• the tilting laser macro already exports a parameter for the calibration reference, so it's straightforward to apply this to all other macros as well.

Thanks for iterating over this with me @Smilels .

[v4hn]

We use this change on our system and to the best of my knowledge this is fully backward-compatible.
If anyone finds problems with this, please report them here and we can deal with them right away.